PublicKey Identifier
A PublicKey identifier is a name for a PublicKey. A PublicKey is a static piece of data. Just as a PublicKey is an excellent name for an entity you can have a conversation with, a SecureHash of a static piece of data is an excellent name for the piece of data. So, a PublicKeyID is just a SecureHash of the PublicKey.
In BirthdayCAKE, the PublicKeyID is of type cake.keystuff.KeyName, and is used consistently whenever a PublicKeyID is required.